So my two biggest pet peeves in OBA systems is leaky fittings, and pressure gauges that don’t read the correct pressure.
I converted to a mechanical pressure gauge not to long ago. I ran an 1/8in air line into the cab into a 1/8 tube to 1/4npt fitting on the back of the gauge. Problem is it reads like 20psi to high. For example it was reading 160psi but the actual tank pressure was 140psi. Not sure what the heck im doing wrong…
Is it substantially warmer in your cab compared to where your tank gauge is? Also, is your in-cab gauge in direct sunlight while your tank gauge is not? I’m asking because:
[ul]
[li]For every 18 °F (10 °C) shift in temperature (from the baseline to which a particular gauge was calibrated), you can expect to experience up to a ±0.4% error (in addition to any error threshold to which the gauge was fabricated/tested). The cause is the change in the elasticity or spring rate of the Bourdon tube element with temperature change.[/li][li]Ambient temperature will cause this – and result in your in-cab gauge being skewed when compared to your on-tank gauge.[/li][li]Direct sunlight will exacerbate this, as gauges contain a good bit of metal and typically sit behind a glass or polycarbonate cover that acts like a lens with regard to sunlight passing through it.[/li][li]Liquid-filled gauges (i.e. those filled with glycerin or silicone oil) generally fare a little better than dry gauges in heat, but they’ll still have distorted readings – it just won’t be as bad.[/li][/ul]Also, I’ve found that standard accuracy pressure gauges seem to be manufactured to accurately read the middle values – i.e. both the upper and lower ends of the scale seem to provide less accurate readings than the middle values of the scale. With this in mind, you might also need to consider the scale of your current gauge compared with the normal/typical values of your air system – and the scale of your tank gauge, if different. If needed, obtain a gauge on which your ‘normal’ values fall in the gauge’s mid-range … and use a gauge with the same scale in both locations to minimize skew.
For Informational/Comparative Purposes Only:
The manual gauge under the hood of my vehicle is 0-300psi for precisely the aforementioned reason, as my 200psi air system’s typical readings fall in the middle of its scale. I also use a digital gauge in the cab and a 0-400psi digital sending unit that takes readings right next to my manual, under-hood gauge … to minimize reading skews between the gauges. I always expect my manual gauge to be less accurate than my digital gauge … but I have both in case one fails … and as long as they’re within 4-8psi of one another I’m happy.
You generally get what you pay for in gauges. If you want accuracy and solid reliability, spend the money for a gauge that will provide it. If you did and aren’t getting that accuracy or reliability, exchange it with the vendor of the gauge – most decent gauge manufacturers will replace faulty gauges free of charge to you. To put that into perspective, my standard accuracy, glycerin-filled, manual, under-hood gauge was nearly $40 USD. I bought it after finding myself completely dissatisfied with a crappy 0-200psi ~$10.00 dry gauge … and a somewhat better 0-200psi ~$20.00 glycerin-filled gauge. As for the digital gauge and sending unit – Dakota Digital’s gauges are downright expensive, but again, you get what you pay for – as they don’t have the problems the Air Zenith digital gauges and sending units have exhibited.
Most gauges are known as “3-2-3” gauges, which means that they are around + or - 2% accurate in the middle of their range, and + or - 3% at first and last 1/4 of their range. Some more expensive gauges are 1% over their full range. Info: http://swhsupply.com/2011/07/the-difference-between-3-2-3-and-1-gauges/